1. Field of the Invention
The present invention relates to dielectric ceramics, and more particularly to dielectric ceramics having high unloaded Q (reciprocal of dielectric loss) in high frequency ranges such as microwaves, millimetric waves, etc.
2. Description of the Prior Art
Generally, high-frequency dielectric ceramics are widely used as dielectric resonators, substrates for monolithic integrated circuits (MICs), etc.
In recent years, the frequency range used has been expanding to a higher range as the satellite broadcasting and satellite communications using the super high frequency band (SHF band) enters an era of practical use, which has spurred the need for the development of dielectric ceramics having a higher unloaded Q.
Ba-Mg-Ta based dielectric ceramics (as disclosed in U.S. Pat. Nos. 4,121,941, 4,487,842, 4,731,207, Japanese Laid-Open Patent Publication No. 61-181008, and Japanese Laid-Open Patent Publication No. 62-170102 have been drawing attention as dielectric ceramics having a high unloaded Q.
High-frequency dielectric ceramics containing a Ba-Mg-Ta based complex perovskite compound as the main component have poor sintering characteristics. Therefore, in the conventional Ba-Mg-Ta system, it has only been possible to obtain a low unloaded Q of 3,740 to 3,920 at the frequency of 11 GHz (U.S. Pat. No. 4,121,941).
In view of the above difficulty, there has been proposed a method which comprises adding Mn as a sintering agent to improve the sintering characteristics (U.S. Pat. No. 4,487,842). This method, indeed, is capable of producing dielectric ceramics having an unloaded Q of 10,000 or larger (at frequency of 11 GHz), but adding such a dissimilar element as Mn not only leads to complication of the manufacturing process but also entails the possibility of causing a detrimental effect on the dielectric properties of the ceramics because of the addition of such an agent.
Furthermore, there has been proposed a special sintering method called a rapid heating method (a method capable of heating up quickly to the desired temperature) aiming at improving the sintering characteristics (U.S. Pat. No. 4,731,207). With this method, it is possible to improve the unloaded Q factor, but because of the difference in thermal contraction between the exterior and interior of the ceramics and other reasons, the reproducibility of unloaded Q is particularly poor, making it difficult to obtain products of stable quality. To deal with this problem, a method has been proposed which comprises covering the compact with refractory powder during heating and sintering (Japanese Laid-Open Patent Publication No. 61-181008), but it cannot be said that this method succeeded in basically solving the problem.
When Ba is denoted as A, Mg as B1 and Ta as B2, for example, the oxide of a complex perovskite structure can be represented by the formula A (B1, B2) O.sub.3.
In dielectric ceramics containing the oxide of a complex perovskite structure as the main component, it is known that when the crystal structure is hexagonal, the structure takes the shape of periodic arrangement (long-range periodic arrangement) repeating over a wide range with three layers as one cycle, i.e. in the order of B1-B2-B2 in the above formula; therefore, the lattice contracts along the direction perpendicular to the (111) direction, causing strain.
It is also said that the unloaded Q factor is improved as the lattice strain is caused, as described above, as a result of the improved regularity of the long-range periodic arrangement. Therefore, in order to improve the regularity and obtain ceramics having high unloaded Q, it has been the usual practice to sinter ceramics prepared from fine powder of a mean crystal grain size of 1 to 2 .mu.m, followed by over-100 hours sintering and over-100 hours heat treatment.
However, such a sintering method has had the disadvantage of an increased sintering cost and a complicated manufacturing process, thus presenting obstacles to the improvement of production efficiency (S. Kawashima et al., J. Am. Ceram. Soc., 66(6) pp421-(1983)).
Also, when the rapid heating method is used to obtain ceramics of high density and heat treatment of extremely long duration is performed to enhance the regularity, it is possible to obtain ceramics of high unloaded Q, but such a technique also has had the shortcomings of complicating the manufacturing process and presenting obstacles to the improvement of production efficiency (Japanese Laid-Open Patent Publication No. 62-170102).